I have used the existing ScrumBurndownPlugin as a model to develop a version including support for re-estimates which we needed. As far as the license info is concerned all of it should be completely free licensing. Please check with ​http://www.fusioncharts.com/free/ .

Installation Instructions

Installing the egg

Install just like you would any other Trac plugin. Drop the egg into your plugins or site-packages folder and then try to access Trac in your browser. Add plugin to [components] section in trac.ini (at least the following plugins must be enabled)

Assign Permissions

Restart The Server

Finally, you will need to restart your webserver in order to see some of the features that BurnDownPlugin provides, such as the addition of a 'burndown' link on the trac navbar.

Using the plugin

You can view different burndown charts, selectable by milestone by choosing from the dropdown and clicking the 'Show Burndown Chart' button.

At the beginning of a milestone (i.e. sprint), make sure the tickets have been inputted and estimated in Trac.
The 'Hours to Add' earliest entered determine the start of the milestone (i.e. sprint) while the one entered last define the end of the milestone.

In order for this to work, it is imperative that developers keep the 'Estimated Hours' and 'Total Hours' fields accurate and up-to-date on the tickets they are working. The 'Total Hours' field itself should never be manually edited, but rather, it can be changed by inputting positive and negative values into the 'Hours to Add' field.

During the spring you can entered revised estimates. The chart display 2 bars per day : one for the total amount of work estimated including revised estimates and a second one displaying the total amount of work performed. Like this you can also see how the total amount of work estimated evolves during the sprint